The present invention relates to an improvement in an exhaust gas recirculation or EGR system for a diesel engine.
It is well known to recirculate exhaust gas in a diesel engine back to the intake in order to reduce the concentration of toxic nitrogen oxides or NOx emitted therefrom.
Such recirculation of engine exhaust tends to produce an increase in the emission of smoke and unburned hydrocarbon or HC. It is therefore necessary to minimize or virtually stop exhaust gas recirculation in a heavy load operating range of the engine wherein the emission of smoke is considerable.
A conventional and typical system to meet the above demand employs means for sensing heavy load operation and cutting off the supply of control vacuum to a vacuum actuator associated with an EGR valve in the specific heavy load operating range, thereby interrupting the recirculation of engine exhaust.
However, although the heavy load condition may be sensed during rapid starting or rapid acceleration of a vehicle to immediately cut off the control vacuum, the exhaust gas recirculation frequently fails to be immediately interrupted. The result is the emission of increased amounts of smoke and unburned HC and CO during rapid starting of the vehicle.
Suppose that the accelerator pedal is depressed from a position corresponding to light load to the maximum depressed position for a quick start of the vehicle. Although this may be immediately responded to by specific means which sense the heavy load condition through the position of a control lever of a fuel injection pump operatively connected to the accelerator pedal, the control vacuum in the vacuum actuator (diaphragm assembly) of the EGR valve still remains therein. The EGR valve does not rapidly close but closes gradually as the control vacuum leaks from the vacuum actuator.
Thus, despite the full depression of the accelerator pedal and the maximum volume of fuel injection, engine exhaust is recirculated back to the intake until the EGR valve fully closes. This is the cause of the sharp increase in the emission of smoke and unburned HC and CO.